The mouse Y-box protein, MSY2, is associated with a kinase on non-polysomal mouse testicular mRNAs

In male germ cells many mRNAs are sequestered by proteins into translationa lly silent messenger ribonucleoprotein (mRNP) particles. These masked pater nal mRNAs are stored and translated at specific times of germ cell developm ent. Little is known about the mammalian testicular mRNA masking proteins b ound to non-polysomal mRNAs. In this report, the major proteins binding to non-polysomal testicular mRNAs were isolated and analyzed. The two predomin ant proteins identified were: a Y-box protein (MSY2), the mammalian homolog to the Xenopus oocyte masking protein FRGY2/mRNP(3+4), and a poly(A) bindi ng protein. A kinase activity was also found associated with these non-poly somal RNAs. The kinase co-immunoprecipitates with MSY2 and phosphorylates M SY2 in vitro. The MSY2 associated kinase is not casein kinase 2, the kinase believed to phosphorylate mRNP(3+4) in oocytes, but a yet unidentified kin ase. MSY2 was found to be phosphorylated in vivo and MSY2 dephosphorylation led to a decrease in its affinity to bind RNA as judged by northwestern bl otting. Therefore, testicular masked mRNAs may be regulated by the phosphor ylation state of MSY2. Reconstitution experiments in which nonpolysomal mRN A-binding proteins are dissociated from their RNAs and allowed to bind to e xogenous mRNAs suggest that MSY2 binds RNA in a sequence-independent fashio n. Furthermore, association of the non-polysomal derived proteins to exogen ous nonspecific mRNAs led to their translational repression in vitro.